This invention describes dispersants for hydrophobic particles and particulate including pigments in water-based inks. Methods of making and using the dispersants are also disclosed.
When hydrophobic particles are placed in water they tend to agglomerate. This tendency makes it difficult to obtain a dispersion of hydrophobic particles in a water-based system. To lessen this agglomeration tendency, a dispersant may be added to the mixture. If the dispersant has both hydrophobic and hydrophilic parts, the hydrophobic part will be attracted to the hydrophobic particle surface and the hydrophilic part will be solvated by the water.
In recent years, the use of thermal ink jet printers in numerous applications has increased dramatically. Such printers use liquid-based inks which are sprayed onto a receptor, typically a sheet of paper or film, to produce an image. By using basic ink colors (black, yellow, cyan, and magenta, or black, red, green and blue, as well as white pigments (such as TiO2)), in various combinations and concentrations, virtually any color may be produced as part of the printed image. Additionally, ink jet technology is well-suited for high resolution graphic images, particularly those produced using electronic printing systems. Such systems typically employ computer technology to create, modify, and store images, text, graphics and the like.
Many of the inks that have been used in the past with ink jet and other printers are primarily comprised of dyes contained within organic-based carrier liquids. Although such inks may offer satisfactory performance in certain applications, the present trend is away from such systems, because such systems tend to produce images that lack the light stability and durability required for outdoor and similarly demanding applications. Additionally, the use of organic-based carrier liquids creates numerous environmental and material-handling complications. Rather, the printing industry has sought inks that are primarily water-based to thereby reduce or eliminate the problems associated with organic solvent-based systems.
Inks may comprise a suspension of pigment particles in a water-based carrier. However, the suspended pigments may tend to agglomerate. Because ink jet printers use very small jet nozzles (on the order of less than about 40 micrometers with droplet volumes on the order of picoliters) to provide high resolution images, pigment agglomerations may restrict or clog the printer heads. This effect is referred to herein as xe2x80x9cplugging.xe2x80x9d Additionally, in the case of thermal ink jet systems, the ink is subjected to high temperatures (approximately 350xc2x0 C.) as a result of the heating element in the nozzle. Typically inks tend to agglomerate at a faster rate at higher temperatures, and may also settle onto, and coat, the heating elements of the printer heads. This causes decreased thermal efficiency of the print head which results in the formation of smaller ink droplets and lower image quality. This effect is commonly referred to as xe2x80x9ckogation.xe2x80x9d To overcome the problems described above, some pigment particles have been stabilized (from agglomeration) with dispersants. In one approach, the dispersants were formed from surfactants having a hydrophilic portion and a hydrophobic portion, which hydrophobic segment adsorbed onto pigment surfaces of varying hydrophobicity. In another approach, copolymers having hydrophilic and hydrophobic segments were used. Examples of these approaches are described in U.S. Pat. Nos. 4,597,794 and 5,085,698.
In the approaches described above, the hydrophobic segments of the surfactant or polymer can adsorb onto pigments by hydrophobic interaction between the dispersant molecule and the organic-based pigments whose surfaces tend to be hydrophobic. This hydrophobic interaction is usually not very strong. Because of this weak attraction, the dispersant molecules may desorb from the pigment surface, thereby allowing the pigment particles to agglomerate. During printing, this can result in plugging of the printer head jet nozzles. Although heat produced in a thermal inkjet system may strengthen the adsorption of the dispersant on the hydrophobic pigment surface, thus enhancing the stability of the ink, plugging and kogation remain a problem.
In view of the foregoing, a need exists for dispersants that strongly adsorb to hydrophobic particles in aqueous media and that inhibit particle agglomeration. A need also exists for water-based inks that offer increased stability. The present invention addresses such needs.One embodiment of the present invention provides a dispersant for dispersing hydrophobic particles in an aqueous system comprising a derivatized dendritic polymer having at least one ionizable moiety and at least one peripheral nonpolar nonpolymerized hydrocarbon hydrophobic moiety. A dendritic polymer having multiple ionizable moieties and multiple peripheral hydrocarbon hydrophobic moieties is preferred.
Another embodiment of the invention relates to stable water-based systems in which the inventive dispersant is incorporated into a water-based dispersion of hydrophobic particles. Collectively, the dispersants have both hydrophobic segments, which can adsorb to the hydrophobic surfaces of the particles, and hydrophilic ionizable segments, which exhibit mutual electronic repulsion thereby inhibiting particles agglomeration. Such dispersion systems comprise an aqueous suspension of a) hydrophobic particles, and b) a water-soluble dispersant comprising a substituted, derivatized dendritic polymer having at least one ionizable moiety and at least one peripheral long chain (i.e., more than 8 carbon atoms) hydrocarbon moiety that can adsorb to hydrophobic particles or particulate.
Other embodiments of the invention comprise a 3-generation or 5-generation hyperbranched polyester polyol having at least one ionizable group, the ionizable group being preferably anionic, and at least one peripheral hydrocarbon group. The hydrocarbon group is preferably a long chain aliphatic group.
The dendritic dispersants of the invention are particularly useful in stabilizing dispersions of hydrophobic materials, including pigments, in liquid dispersions and suspensions, especially aqueous dispersions and suspensions. The ionizable groups on the compact surface of a hyperbranched polymer provide a high charge density in a polar solution such as water. This high charge density greatly enhances interparticle repulsion, thereby helping to stabilize the dispersion.
A particular embodiment of the present invention relates to stable, water-based pigmented inks that meet the stringent requirements of ink jet and other printing applications. In these inks an inventive dispersant is incorporated into a water-based pigment dispersion to eliminate or reduce the likelihood that the hydrophobic pigment particles will agglomerate. Such inks comprise, generally, an aqueous suspension of a) pigment particles, and b) a dispersant comprising a dendritic polymer having at least one ionizable moiety and at least one peripheral hydrophobic hydrocarbon moiety that can adsorb to the pigment particles. When used to disperse hydrophobic pigment particles in an ink dispersion, the dispersant may enhance the colloidal stability of the ink, particularly under high temperature conditions. The dispersants may also provide a decrease in viscosity of the ink.
In another embodiment the dispersants of the invention may be used as binders to disperse hydrophobic particles of, e.g., silica, titania, or zirconia, in aqueous media to form suspensions. The suspensions may then be coated onto a substrate to form a film by conventional coating methods such as gravure, roll, curtain, die, and coextrusion.
The dispersants of the invention may be used alone or in conjunction with another dispersant as a primary or secondary dispersant. Inks containing the dispersants of the invention as a co-dispersant may provide prints with superior quality in terms of uniformity of color, absence of any banding, and adhesion to the printing substrate.
As used in this invention:
xe2x80x9cadsorbxe2x80x9d refers to the adherence of an atom, ion, or molecule on the surface of another substance;
xe2x80x9cbandingxe2x80x9d refers to visible striations or lines of lower optical density that can occur when an ink is printed on a substrate;
xe2x80x9cderivatizexe2x80x9d or xe2x80x9cderivatizationxe2x80x9d refers to producing or obtaining a material from a precursor material by chemical reaction(s) that adds or modifies a functional group on the precursor material;
xe2x80x9cdispersant/particle compositexe2x80x9d means a dispersant molecule and the particle or particulate to which the dispersant is adsorbed; typically the composite will comprise layers of dispersant molecules with the particle;
xe2x80x9caliphatic group,xe2x80x9d xe2x80x9caliphatic moiety,xe2x80x9d or xe2x80x9clong chain aliphatic group or moietyxe2x80x9d means a linear, branched, cyclic, unsubstituted or substituted, non-polar hydrocarbon having approximately 6 to 100, preferably 8 to 20, carbon atoms;
xe2x80x9caromatic groupxe2x80x9d or xe2x80x9caromatic moietyxe2x80x9d means a nonpolar hydrocarbon, having approximately 6 to 100, preferably 8 to 20, carbon atoms, comprising at least one cyclic structure containing at least one xcfx80-conjugated ring structure;
xe2x80x9chydrocarbon groupxe2x80x9d or xe2x80x9chydrocarbon moietyxe2x80x9d means an aliphatic or aromatic group or moiety;
xe2x80x9cionizablexe2x80x9d means a neutral molecule comprising atoms capable of losing or gaining electrons, thereby acquiring a net electrical charge and becoming an ion, e.g., when placed in a polar medium such as water;
xe2x80x9cmonodispersexe2x80x9d refers to particles having the same or materially the same sizes or shapes, or polymer molecules having materially the same molecular weights;
xe2x80x9cpolydispersexe2x80x9d refers to particles having various sizes or shapes, or polymer molecules having a distribution of molecular weights;
xe2x80x9cperipheralxe2x80x9d means the terminal end of any branch or stem on a dendritic polymer;
xe2x80x9chydrophobic moietyxe2x80x9d or xe2x80x9chydrophobic segmentxe2x80x9d means a nonpolar, water-insoluble hydrocarbon moiety that can associate or bond with the surface of a hydrophobic particle; a hydrophobic segment is not polymerized, but rather derived from a single aliphatic or aromatic precursor, e.g., n-octyl mercaptan;
xe2x80x9chydrophobic particlexe2x80x9d means a particle that is nonpolar or has a nonpolar surface;
xe2x80x9csolvatexe2x80x9d refers to the phenomenon wherein a strong affinity of a solvent, e.g., water, for particles of dissolved or suspended substances leads to the particles acquiring a tightly held film of solvent on their surfaces; and
xe2x80x9cwater solublexe2x80x9d refers to a material capable of being homogeneously distributed in water, such as a material that will form an aqueous solution or dispersion.
Further aspects of the invention will become apparent with reference to the following Figures, Detailed Description, and Claims.